There has been known a method for indirectly detecting whether a tire has a decreased pneumatic pressure by making a relative comparison among angular velocities of tires attached to a vehicle (see Patent Literatures 1 and 2 for example).
According to these methods, running statuses of a vehicle are limited (to a status where the vehicle is running at a fixed speed on a straight flat road). The methods make a relative comparison among the wheel speeds of the four wheels under the above running status. When only one wheel of the four wheels shows an increased wheel speed, these methods determine that the increased wheel speed is caused by the fact that a dynamic loaded radius (DLR) of the tire is reduced due to a decreased pressure, then issue an alarm.
The relative comparison is carried out by using a decreased pressure determination value (DEL) as shown in the following formula (1) for example. When this decreased pressure determination value exceeds a predetermined threshold value, these methods determine that a decreased pressure is caused.DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)  (1)
In the formula, F1 to F4 represent the angular velocities of a left-front wheel, a right-front wheel, a left-rear wheel, and a right-rear wheel, respectively.
In this case, there are shown no clear criteria that clarify how much reduction of the dynamic loaded radius (i.e., how much increase of the rotation number) leads the determination of a decreased pressure. Thus, a decreased pressure is determined when an appropriately-set threshold value is exceeded.